Boiler stops working after a few minutes

It's our very old vented ch system with a Potterton neataheat boiler. Having attempted some system maintenance the boiler cuts out after a few minutes presumably because the hot water doesn't seem to be circulating.
Sorry about the long post but I know you guys like to know all the details before suggesting any action.
The system had been pumping over for some time and it was suggested that this may be due to a blockage around the junction of the 15mm feed from the expansion tank to the 22mm flow.
We drained it down, cut out and replaced the pipework from 6" below the feed, past the vent and up to the pump, including a few inches of the vent and feed pipes.
A few days Before starting I'd added Sentinel 400 to help get the crud moving.
We then hired a power flusher, removed a radiator to plug it in, capped off the feed and vent pipes, removed the motor unit from the 3-way valve and set to CH, and cycled with water through all rads and individual adds for some hours, reversing the flow and dumping the water regularly. Then added Sentinel 800, cycled that for a couple of hours through all rads and through each rad for 10 mins. Finally flushed clean water through everything until it was clear.
A valve at the pump was closed for much of the to force water through other rads, but ran with the valve open for some of the time. The removed rad will be replaced with a new plinth heater.
We then reconnected the feed and vent, bled all the rads and pump a couple of times. Switched on and bled everything again.
I removed the cap on the pump and its spinning well when the system is running.
When the boiler runs the water doesn't seen to circulate - the flow and return above the boiler are similar temperatures and the flow to the pump is cold.
Cam anyone suggest what might be wrong please?

Quote - "A valve at the pump was closed for much of the..."
The clue was in the post. Daft old pl**ker. I was so sure I'd turned that valve back on that I didn't recheck it. Either my mind's gone or I shouldn't have been flushing through the CH system until 5am this morning!

hi
i may have missed it in your post, but is sounds like to main heat exchanger is blocked or the pumps not working.
the pump may 'spin' but i have seen pumps with the impeller all broken up.
try to take the pump head off to check the impeller.
also check the resistance on the flow thermistor say around 10K ohms at room temp.
a faulty thermistor will fool the boiler into thinking the water is hot.

Thanks for your reply Peter, I did wonder whether I'd managed to damage the pump impellers with chunks of crud loosened in the system or cause some damage to the boiler, but turns out it was just me being dozy and failing to turn an isolator valve back on at the pump.
System is now purring along happily, rads are nice and uniformly hot, water in the expansion tank is cold and I know a lot more about my CH system that I did on Friday - shame that at my age I'll have forgotten most of it by Tuesday.
Cheers.